This invention relates to fuel cells and, in particular, to an improved electrolyte for use in molten carbonate fuel cells.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Molten carbonate fuel cells (MCFCs) operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. The anode and the cathode of MCFCs are isolated from one another by a porous electrolyte matrix which is saturated with carbonate electrolyte. Typical MCFC designs include carbonate electrolyte stored in the pores of the anode and of the cathode and in gas passages formed in the anode and cathode current collectors. Generally MCFCs operate at intermediate temperatures of 575° C.-670° C., and the electrolyte melts during the initial heat up of the fuel cell and redistributes among the pores of the anode, the cathode and the electrolyte matrix due to the capillary forces of the pores. Conventional MCFCs typically use a eutectic carbonate mixture as the carbonate electrolyte, such as a eutectic mixture of 62 mol-% lithium carbonate and 38 mol-% potassium carbonate (62% Li2CO3/38% K2CO3) or a eutectic mixture of 52 mol-% lithium carbonate and 48 mol-% sodium carbonate (52% Li2CO3/48% Na2CO3).
It is generally recognized that the Li/Na carbonate mixtures have higher ionic conductivity, lower vapor pressure and cause lower cathode dissolution, than the Li/K carbonate mixtures. However, a major disadvantage of the Li/Na carbonate mixtures is a lower oxygen gas solubility, which significantly affects the performance of fuel cells at lower temperatures (T<600° C.). That is, fuel cells using the 52% Li2CO3/48% Na2CO3 electrolyte exhibit lower performance than the fuel cells using 62% Li2CO3/38% K2CO3 electrolyte due to high cathode polarization which is caused by low oxygen solubility, particularly at lower operating temperatures. As a result, in order to effectively use Li/Na carbonate electrolyte at MCFCs, the performance of the Li/Na carbonate electrolyte needs to be improved at low operating temperatures of T<620° C. and the surface tension of the electrolyte needs to be reduced so as to improve gas solubility therein.
It has been reported in literature that the addition of certain additives that include Rb and/or Cs to the carbonate melt lowers its surface tension because the Rb and Cs ions are larger than the Li and Na ions and thus Rb and Cs act as surfactant to decrease the surface tension of the Li/Na carbonate electrolyte and to improve gas solubility. In addition, DE10136156 to Hoffman Joachim discloses use of Li/Na/Cs, Li/K/Cs, Li/Na/Rb or Li/K/Rb or mixtures thereof as electrolytes for MCFS, and that the use of the Li/Na/Cs electrolyte, formed by mixing 52% Li/48% Na and 70% Li/30% Cs carbonates, improved the performance of the cathode at 650° C. due to low polarization over the performance of the eutectic 62% Li/38% K carbonate electrolyte. Applicants in the present invention performed tests on cells that used the 52% Li/48% Na carbonate electrolyte doped with Cs and/or Rb as disclosed in the DE10136156 patent, and found that these cells exhibited lower performance at lower temperatures T<620° C. than fuel cells using 62% Li/38% K carbonate electrolyte, because of high cathode loss and high mass transfer resistance.
U.S. Pat. No. 5,942,345 discloses that Li/Na carbonate electrolyte doped with small amounts of CaCO3 and BaCO3, preferably in equimolar amounts, provides better performance than the eutectic 52% Li/48% Na mainly at low temperature. However, when Li/Na carbonate electrolytes doped with BaCO3 and CaCO3 were tested by applicants in button cells (3 cm2), the addition of the Ba and Ca additives only showed a minor effect on the performance of the cathode, and the overall cell performance at low temperatures of T<600° C. remained lower than that of fuel cells using Li/K carbonate electrolyte. In particular, button cell tests showed that the cathode polarization in cells with Li/Na carbonate electrolytes doped with Ba and Ca was two times higher than cathode polarization in cells using Li/K carbonate electrolyte. In addition, applicants' tests performed in single cells (250 cm2) with Li/Na/Ba/Ca carbonate electrolytes showed low performance at T<600° C. due to high cathode polarization.
It is therefore an object of the present invention to provide an improved Li/Na carbonate electrolyte for use in MCFCs that has improved performance over the Li/K carbonate electrolyte at low MCFC operating temperatures of T<620° C.
In particular, it is an object of the present invention to provide an improved Li/Na carbonate electrolyte that has lower surface tension and improved gas solubility and that reduces polarization of the cathode at low MCFC operating temperatures of T<620° C.
It is also an object of the present invention to provide a method of storing the improved electrolyte in the fuel cell components so as to provide for the improved performance.